1. Field of the Invention
The present invention relates to porous hollow fiber membranes for the separation of plasma, that is to plasmapheresis of blood and in particular of human blood.
2. Brief Description of the Background of the Invention Including Prior Art
The medical research is to an increasing degree interested in the question of the nature of the constituents of the blood in order to be able to improve the therapeutical methods of treatment. In this context both the quantitative as well as the quantitative composition of the blood is of interest as well as their changes, be it caused by sickness or other influcnces and of interest are also the changes of composition caused by elimination processes such as for example by way of hemodialysis.
The hemodialysis by way of the artificial kidney has experienced an incomparably strong increase over the last fifteen years and the hemodialysis has lengthened considerably the live of nephropathic persons as well as of those who have lost for one reason or another their kidneys. Many persons owe their being alive solely to the functioning of a membrane for the elimination of metabolic decomposition products from the blood by way of dialysis.
These chemically prepared semipermeable membranes, the most important of which is the cuprophane membrane comprising regenerated cellulose, imitate the separation process of natural membranes within the human body in an approximate way to such an extent that these chemically prepared semipermeable membranes are capable of substituting for a non-operating, sick or underfunctioning organ.
The concentration difference of the so-called metabolites between the blood side and the water side of the membrane induces a steady diffusion of the substance from the blood, and the molecular size of the substance diffusing is determined by the kind of porosity of the dialysis membrane The same principle prevails relative to an infinite number of cell and organ membranes within the human body.
The nephrologists have learned to operate in this context the process of extracorporeal blood circulation, that is the extension of the human blood circulation to a branch outside of the human body and they have created thereby the basis for a further reaching elimination process such as the hemofiltration.
In the hemofiltration by way of an artificial kidney the metabolites are removed by way of a membranein such a way that the filtrate containing the metabolites having passed through the membrane is discarded in large volumes such as for example 20 liters. The large amount of filtrate volume has to a large extent to be substituted by a physiological sodium chloride solution in order to protect the blood from an intolerable thickening.
In both cases, that is with the hemodialysis and with the hemofiltration the membrane is not allowed to pass protein materials, the most common representative of which is albumin (molecular weight about 68,000) and that means the membrane has to be tight for this size of molecule.
Medical research has recognized that many illness are caused by toxins which are frequently attached to proteins. The toxins themselves are frequently small molecules, which would pass freely the mentioned dialysis and hemofiltration membranes if the toxins were present free and unattached.
By way of the protein attachment, however, they represent molecules of such size which cannot be eliminated by the above mentioned membranes and to these proteins belong also the immunocomplexes and the antigens.
In order to remove such protein-attached toxins with a molecular weight between from about 100,000 to 3,000,000 there existed in the past only the process of plasma-separation by way of the ultracentrifuge. An allowable amount of blood was removed from the patient into a bag, the contents of the bag was filled into the ultracentrifuge and the cellular parts were separated from the blood plasma. The cellular components such as red and white blood cells as well as blood platelets are returned to the patient after thinning and the plasma including the dissolved components is discarded and in this way the toxic materials are eliminated.
The same process is inter alia also presently still employed with healthy blood donors in order to obtain plasma for the purpose of the plasma exchange with patients.
The blood separation into cells and plasma therefor has two different areas of application, that is the elimination of protein bound toxins and the production of plasma for plasma exchange.
It would be desirable for the two cited areas of application to employ an extracorporeal hemofiltration at the patient or donor, which would allow to separate the plasma still containing all dissolved materials and returning directly to the patient or donor the fraction containing the cell components of the blood into the blood circulation. This however is preconditioned on the existence of membranes which have a permeability limit only at a molecular weight of about 3 million.
There are a few types of membranes, which are employed for this purpose such as the cellulose acetate the cellulose nitrate membrane and the polyvinylalcohol membrane. These however have only a limited or better called partial permeability for the recited proteins. The permeability can be expressed by way of the so-called filtration coefficient ##EQU1## wherein C.sub.F =concentration of the component X in the filtrate
C.sub.B =concentration of the component X in the blood.
The determination of the filtration coefficient can be performed for example by way of laser-nephelometric analysis.
If S=1, then a complete permeability is present if S is smaller than 1 then a partial permeability is present. The presently available membranes have filtration coefficients, which are already for albumin less than 0.8. This means that the elimination of the components to be separated takes place only in a diminished degree, therefor a longer treatment time is required. Also, one has to work with larger amounts in backinfusions or larger membranes have to be employed, which carries considerable disadvantages since such modules then exhibit a very large extracorporeal volume.